1. Field of the Invention
This invention relates generally to circuits having a nonlinear response characteristic and, more particularly, to a circuit having a nonlinear response characteristic for use in providing gamma compensation in a video camera.
2. Description of the Prior Art
It is well known that a color picture tube has a nonlinear transfer characteristic, however, the pick-up tube in a video camera has essentially a linear response characteristic. Therefore, in order to produce a pleasing color picture, the transfer characteristic of the pick-up camera must be compensated in the reciprocal manner relative to the picture tube characteristic. Such compensation is typically referred to as gamma compensation, in which gamma is understood to be the slope of a log-log plot of the light transfer characteristic.
Gamma-compensation circuits are designed using a number of series-connected diodes to provide the required nonlinear response characteristics, in which the principal that the voltage across a diode is approximately proportional to the square root of the current through such diode is advantageously employed. Some known gamma-compensation circuits are temperature dependent and, thus, this characteristic presents a drawback to the use of such circuits. Other known gamma compensation circuits have a direct current component in the nonlinear transfer function, which adversely affects the stability of the gamma compensation circuit.
Other known techniques for constructing gamma-compensation circuits employ using several gamma-compensating circuits in series, so that different portions of the nonlinear curve can be provided by different, individual, compensation circuits. Nevertheless, matching of these circuits is quite difficult and this technique has generally been not effective.